Might be a silly question, but have you got any kind of chip device installed, a friend of mine experianced similar problems on his sorrento and it turned out to be a faulty chip he had installed in line with the coil for some extra umph
I would have thought it would produce a fault code if it goes into limp home mode. Does it remain in limp home mode or doe it clear when things level off? If it is not an intermittent fault, find a hill close to the dealer. Make sure it does it before on the day, then drag a technician out from the dealer and let them see it first hand.